In the screw feeding device, a nut member is threadedly engaged with a screw shaft having helical grooves formed on an outer peripheral surface. The screw feeding device has a function to convert a rotational motion of the screw shaft into a linear motion of the nut member. Therefore, the screw feeding device is suitable for use in converting the rotational motion of an electric motor into the linear motion. For example, the screw feeding device is employed for an actuator configured to output the rotation of the electric motor as an advancing and retreating motion of a rod member. In this actuator, in order to efficiently convert the rotational motion of the electric motor into the advancing and retreating motion of the rod member, a ball screw device, in which rolling elements such as balls are interposed between the screw shaft and the nut member, is employed as the screw feeding device. In this actuator, an amount of advancement and retreat of the rod member is determined in accordance with a rotation angle of the electric motor.
Meanwhile, depending on the usage of the actuator, there is a case where other members are desired to be elastically pressed by the rod member, or a case where the impact load, which acts on the rod member from outside, is desired to be reduced without acting on the screw feeding device. In Patent Literature 1, there is disclosed a screw feeding device, in which a buffer mechanism is provided between the nut member and the rod member. In the screw feeding device disclosed in Patent Literature 1, the rod member includes a piston configured to receive the nut member, whereas a pair of spring holders is fitted to an outer peripheral surface of the nut member. The pair of spring holders is fitted to a groove formed in the outer peripheral surface of the nut member, and is freely movable in the groove in an axial direction. Further, a spring is provided between the pair of spring holders under a compressed state. The piston of the rod member has an engaging flange positioned on an outer side of the pair of spring holders. When the rod member advances or retreats in the axial direction, the engaging flange interferes with any one of the spring holders so that the spring holder moves in a direction of compressing the spring.
In the screw feeding device disclosed in Patent Literature 1, even when any one of a compressive load or a tensile load acts between the nut member and the rod member, the engaging flange of the rod member presses the spring holder in the direction of compressing the spring so that the compressive load acts on the spring. Accordingly, when the compressive load exceeding an elastic force exerted by the spring acts on the spring, the spring is compressed so that the other members are elastically pressed by the rod member. In addition, the impact load acting on the rod member from outside can be reduced without directly acting on the screw feeding device.